The present invention relates to electronic circuits, and more particularly, to techniques for on-chip termination on electronic circuits.
When transmitting signals over distances that are appreciable with respect to the signal period, mismatches between the impedance of the transmission line and that of the receiver may cause signal reflection. The reflected signal interferes with the transmitted signal and causes distortion and degrades the overall signal integrity. To minimize or eliminate the unwanted reflection, transmission lines are resistively terminated by a matching impedance. In the case of integrated circuits that are in communication with other circuitry on a circuit board, termination is often accomplished by coupling an external termination resistor to the relevant input/output (I/O) pins.
For many high speed integrated circuits, and particularly those that have large I/O pin counts, external termination poses a number of problems. A termination resistor is typically coupled to every I/O pin receiving an input signal from a transmission line. Often hundreds of termination resistors are needed for an integrated circuit. Numerous external termination resistors can consume a substantial amount of board space. The use of external components for termination purposes can be cumbersome and costly, especially in the case of an integrated circuit with numerous I/O pins.
Signal integrity is crucial in digital design. To improve signal integrity, both single-ended and differential signals are terminated. Termination can be implemented with external termination resistors on a board or with on-chip termination technology. On-chip termination eliminates the need for external resistors and simplifies the design of a circuit board. It is therefore desirable to implement the termination resistance on-chip to reduce the number of external components.